NEW YORK (Reuters) - Norwegian Air Shuttle ASA <NWC.OL> will start flying four new routes between the United States and Europe, the budget carrier said on Thursday, rapidly expanding its low-cost transatlantic flights and increasing pressure on established carriers.

Starting next summer, Norwegian will operate flights to Amsterdam, Madrid and Milan from New York's John F. Kennedy International and Los Angeles International airports.

These add up to 25 new U.S. routes for Norwegian this year alone, and 11 more are already planned for 2018.

Norwegian's rapid growth in the U.S. market since being granted flying rights by the outgoing Obama administration in 2016 has rankled legacy carriers. They complain that it undermines wages and degrades working standards, but Norwegian has dismissed those claims.

Established U.S. and European airlines have been forced to reassess the way they court customers as budget airlines offer cheap tickets for relatively comfortable flights on newer, sleeker jets.

Among U.S. airlines in particular, the fight for both low and big budget travelers faces added challenges. A pricing squeeze from low-cost carriers like Norwegian has pushed down fares, while higher-end airlines like Emirates [EMIRA.UL] have raised customer expectations for comfort.

Including taxes, introductory fares for nonstop Norwegian flights to both Madrid and Milan from Los Angeles start at $229 for a one-way, main cabin seat. Nonstop flights to Amsterdam and Madrid start at $199 and $229 one-way from New York.

Summer fares listed on Google Flights on these routes are more than 10 times as expensive on other carriers.

On the Los Angeles-Milan route, no other nonstop flights from other airlines are listed during that time, and Norwegian will be the only airline operating year-round nonstop flights from Los Angeles to Madrid and Milan.